Bioreactors
Tissue Culture * One approach to engineer tissues involve the in vitro culture of cells in scaffolds to generate functional tissues for in vivo applications ** An engineered tissue should provide some minimal level of function that should improve progressively in vivo until normal tissue function has been restored ** Engineered tissues should have similar cellular structure and function as the targeted tissue ** Engineered cartilage should provide a low-friction, articulating surface and be able to withstand and transmit loading in compression, tension, and shear ** Engineered heart muscle should conduct electrical signals, contract in a coordinated manner, and withstand dynamic changes in pressure, tension, and shear ** Bioreactors with appropriate stimuli (mehcanical, electrical) are necessary to allow the engineered tissues to perform these functions Bioreactors * In vitro culture system designed to perform some or all of the following functions ** Provide control over the initial cell distribution on a the 3D scaffolds ** Provide efficient mass transfer of oxygen, nutrients, and regulatory factors to tissue engineered constructs during the in vitro culture ** Expose the developing cell/scaffold constructs to mechanical or other stimuli Types of Bioreactors Tissue engineering bioreactor * Designed to precisely regulate the cellular microenvironment in order to facilitate construct uniformity and overall cell viability * Key requirements ** Increased cell seeding efficiency into the 3D scaffold ** Improved mass transfer ** Adequate gas exchange\ ** Temperature and pH control ** Physiological stimuli Non-specific tissue engineering bioreactor * Aimed at optimizing process variables prior to actual tissue engineering applicaitons * They are utilized primarily for screeening purposes and are particularly advantageous for investigating the physiological parameters Considerations When Designing/Using Bioreators * Mass Transport Considerations ** Nutrients,oxygen, and regulatory molecules have to be efficiently transported from the bulk culture medium to the tissue surfaces and then through the tissue to the cells ** Metabolites and carbon dioxide have to be removed from the cells through the tissue matrix to the surfaces and then to the bulk medium ** For example, tissue engineered heart tissue requires a bioreactor to allow for cell growth *** Heart tissue is about 13mm thick *** It is hard for nutrients to penetrate into the middle of the scaffold ** If cells don't receive these nutrients they will die * Biophysical Signaling Considerations ** Tissues and organs in the body are subjected to complex biomechanical environments (dynamic stresses and strains) ** Biophysical signals that play a role in cell physiology in vivo can also modulate the activity of cells within engineered tissues cultured in vitro ** To generate functional tissues, one or more regimes of controlled physical stimuli have to be applied through bioreactors to 3D-engineered constructs in an attempt to improve or accelerate the generation of a functional tissue Bioreactors for Cell Seeding * Cell seeding of a scaffold is the first step in tissue engineering and plays a critical role in determining subsequent tissue formation. * High and spatially uniform initial cell densities are necessary during tissue development * Spinner Flask Seeding ** Can improve efficiency and spatial uniformity of cell seeding throughout porous solid 3D scaffolds as compared with controls seeded statically * Perfusion Cell Seeding ** Cell suspension directly penetrate though a porous solid 3D scaffold via unidirectional or bidirectional perfusion Static Culture vs. Bioreactor Culture * Static Culture: Immerse cell-scaffold constructs in culture medium, medium diffuses into the scaffold to provide nutrients and oxygen to the cells, due to limited diffusion ability, results in constructs with about 100 um thick outer layer of viable cells, no cells presented in the middle of the scaffold ** Limits the maximum thickness to 200 um * Construct culture in bioreactors allow nutrient and oxygen transport more efficiently than static culture, leading to more efficient growth and distribution in the scaffold